This invention relates to rubber-reinforced copolymers, particularly to copolymers of monovinylidene aromatic and unsaturated nitrile having a rubber which exhibits a high solution viscosity dispersed as small rubber particles throughout the copolymeric matrix, and to a method for preparing the rubber-reinforced copolymer.
Rubber-reinforced (or modified) copolymers of a monovinylidene aromatic such as styrene and an unsaturated nitrile such as acrylonitrile having particulates of rubber, generally an alkadiene rubber, dispersed throughout the copolymeric matrix (conventionally referred to as ABS resins) are employed in a wide variety of commercial applications such as packaging, refrigerator linings, furniture, domestic appliances and toys. It is well known that the physical properties of an ABS resin such as toughness (i.e., the combination of elongation and impact strength), at both room and lower temperatures, are affected by the size, composition and morphology of the dispersed rubber particles and/or the concentration of rubber in the rubber-reinforced copolymers. For example, to achieve the balance of physical properties required in many applications, the rubber particles are necessarily dispersed through the copolymer matrix at a relatively small size, e.g., less than about 1.5 micron.
Heretofore, ABS resins have been prepared using a variety of polymerization techniques. For example, in one method for preparing an ABS resin, preformed rubber particles are admixed with styrene/acrylonitrile prior, during or subsequent to the polymerization of said monomers. In general, emulsion polymerization techniques are employed to prepare the preformed rubber particles which then comprise dense, rubber droplets of coagulated latex. Using said techniques, desirably small rubber particles can be prepared from essentially all rubbers which can be prepared using emulsion polymerization techniques. Unfortunately, significant costs are incurred in the preparation of the rubber particles using emulsion polymerization techniques and the emulsion polymerization aids such as the emulsifiers often have an undesirable effect on the properties of the resulting ABS resin product. Moreover, a relatively high concentration of the emulsion polymerized rubber particles is generally required to impart the desired balance of properties to the resulting rubber-reinforced copolymer product.
An alternative, more econcomical, method for preparing an ABS resin consists of mass polymerization techniques. Such techniques involve dissolving the rubber in a mixture of the styrene and acrylonitrile monomers and subsequently polymerizing the monomers. The resulting copolymer initially forms a discrete, discontinuous phase until, after sufficient polymerization, the rubber separates as a plurality of particles dispersed through a continuous phase now containing the copolymer (so-called "phase inversion"). Polymerization is completed and any solvent or unreacted monomer thereafter removed from the resulting product. (See, for example, U.S. Pat. No. 3,243,481). A similar process involves a combination of mass with suspension polymerization. In said method, following phase inversion, the polymerization mixture is suspended in water and polymerization completed. (See, for example, U.S. Pat. No. 3,509,237). Although an ABS resin can economically and effectively be prepared using either mass or mass/suspension polymerization techniques, ABS resins having the desired balance of physical properties have not heretofore been prepared using conventional mass or mass/suspension polymerization techniques with certain high molecular weight rubbers such as an essentially linear, high molecular weight, low-cis homopolymer of butadiene. Specifically, although the particle size of dispersed rubber can be reduced by increasing the rate of mechanical agitation in the polymerization reactor, an ABS resin wherein the high molecular weight rubber exhibits a desirably small particle size (i.e., a volume average particle size of less than 1.5 micron) at a sufficiently high concentration to impart the desired properties to the final product has not previously been prepared using mass or mass/suspension polymerization techniques.
Therefore, in view of the stated deficiencies of the ABS resins and the method for their preparation disclosed in the prior art, it remains highly desirable to provide an effective and economical method for preparing a rubber-reinforced copolymer of a monovinylidene aromatic and an unsaturated nitrile having discrete particles of a high molecular weight rubber dispersed throughout the copolymer matrix using mass or mass/suspension polymerization techniques.